Marchisio has a whole different role in 3-5-2 .
In 4-3-3 we had only one striker and 2 wide player on every side ( wingers and side defenders), So he was playing box to box and moving much closer to goal to cover the space behind our only forward. Now in 3-5-2 we have 2 forwards but only 1 wide player on every side. Now he is playing much deeper and closer to our box covering the space behind wingers and also creating space for Pirlo with his movement and support.
An example of Marchisio movement in the field in 4-3-3, His heat map in 2-0 win against Cesena which we had 29 shots and he scored one :
Last night game against Catania, we also had 29 shots and here is Marchisio heat map :
* Personally I don't think Marchisio is playing worse than before, he is just doing what coach is asking him.